It is known in the art to use heat treatable aluminium alloys in a number of applications involving relatively high strength such as aircraft fuselages, vehicular members and other applications. Aluminium alloys 6061 and 6063 are well known heat treatable aluminium alloys. These alloys have useful strength and toughness properties in both T4 and T6 tempers. As is known, the T4 condition refers to a solution heat treated and quenched condition naturally aged to a substantially stable property level, whereas T6 tempers refer to a stronger condition produced by artificially ageing. These known alloys lack, however, sufficient strength for most structural aerospace applications. Several other Aluminium Association (“AA”) 6000 series alloys are generally unsuitable for the design of commercial aircraft which require different sets of properties for different types of structures. Depending on the design criteria for a particular aircraft component, improvements in strength, fracture toughness and fatigue resistance result in weight savings, which translate to fuel economy over the lifetime of the aircraft, and/or a greater level of safety. To meet these demands several 6000 series alloys have been developed.
European patent no. EP-0173632 concerns extruded or forged products of an alloy consisting of the following alloying elements, in weight percent:                Si 0.9-1.3, preferably 1.0-1.15        Mg 0.7-1.1, preferably 0.8-1.0        Cu 0.3-1.1,preferably 0.8-1.0        Mn 0.5-0.7        Zr 0.07-0.2, preferably 0.08-0.12        Fe <0.30        Zn 0.1-0.7, preferably 0.3-0.6        balance aluminium and unavoidable impurities (each <0.05, total <0.15).The products have a non-recrystallised microstructure. This alloy has been registered under the AA designation 6056.        
It has been reported that this known AA6056 alloy is sensitive to intercrystalline corrosion in the T6 temper condition. In order to overcome this problem U.S. Pat. No. 5,858,134 provides a process for the production of rolled or extruded products having the following composition, in weight percent:                Si 0.7-1.3        Mg 0.6-1.1        Cu 0.5-1.1        Mn 0.3-0.8        Zr <0.20        Fe <0.30        Zn <1        Ag <1        Cr <0.25        other elements <0.05, total <0.15        balance aluminium,and whereby the products are brought in an over-aged temper condition. However, over-ageing requires time and money consuming processing times at the end of the manufacturer of aerospace components. In order to obtain the improved intercrystalline corrosion resistance it is essential for this process that in the aluminium alloy the Mg/Si ratio is less than 1.        
U.S. Pat. No. 4,589,932 discloses an aluminium wrought alloy product for e.g. automotive and aerospace constructions, which alloy was subsequently registered under the AA designation 6013, having the following composition, in weight percent:                Si 0.4-1.2, preferably 0.6-1.0        Mg 0.5-1.3, preferably 0.7-1.2        Cu 0.6-1.1        Mn 0.1-1.0, preferably 0.2-0.8        Fe <0.6        Cr <0.10        Ti <0.10        the balance aluminium and unavoidable impurities.The aluminium alloy has the mandatory proviso that [Si+0.1]<Mg<[Si+0.4], and has been solution heat treated at a temperature in a range of 549 to 582° C. and approaching the solidus temperature of the alloy. In the examples illustrating the patent the ratio of Mg/Si is always more than 1.        
U.S. Pat. No. 5,888,320 discloses a method of producing an aluminium alloy product. The product has a composition of, in weight percent:                Si 0.6-1.4, preferably 0.7-1.0        Fe <0.5, preferably <0.3        Cu <0.6, preferably <0.5        Mg 0.6-1.4, preferably 0.8-1.1        Zn 0.4 to 1.4, preferably 0.5-0.8        at least one element selected from the group:                    Mn 0.2-0.8, preferably 0.3-0.5            Cr 0.05-0.3, preferably 0.1-0.2                        balance aluminium and unavoidable impurities.The disclosed aluminium alloy provides an alternative for the known high-copper containing 6013 alloy, and whereby a low-copper level is present in the alloy and the zinc level has been increased to above 0.4 wt. % and which is preferably in a range of 0.5 to 0.8 wt. %. The higher zinc content is required to compensate for the loss of copper.        
In spite of these references, there is still a great need for an improved aluminium base alloy product having improved balance of strength, fracture toughness and corrosion resistance.